Hitherto, as a recording medium for information signals, disc-shaped optical recording media such as optical disc or magneto-optical disc have been proposed. Further, optical pick-up devices for carrying out write and read operations of information signals with respect to such disc-shaped optical recording media have been proposed. In addition, disc players caused to be of structure comprising such optical pick-up and adapted for carrying out recording and reproduction of information signals with respect to disc-shaped optical recording media as described above have been proposed.
Such a disc-shaped optical recording medium is caused to be of structure including a transparent base (substrate) consisting of transparent material such as polycarbonate, and a signal recording layer deposited and formed on one principal surface portion of the transparent base. The optical pick-up is caused to be of structure including a semiconductor laser serving as a light source, an object lens (objective) to which light beams emitted from the semiconductor laser are incident, and a photo-detector.
The light beams incident to the object lens are irradiated, in converged state, onto the signal recording surface of the disc-shaped optical recording medium by the object lens. In this case, these light beams are irradiated from the transparent base (substrate) side of the disc-shaped optical recording medium onto the disc-shaped optical recording medium, and are converged onto the signal recording surface which is the surface portion of the signal recording layer after transmitted through the transparent base. This object lens is caused to undergo movement operation in the state supported by the bi-axial actuator to thereby converge the light beams onto the portion where information signal is recorded on the signal recording surface, i.e., a predetermined recording track. Such recording tracks are formed so as to take spiral shape on the principal surface portion of the disc-shaped optical recording medium.
In the disc-shaped optical recording medium, light beams are irradiated in the state converged after passed through the object lens, whereby write or read operation of information signal is carried out at the portion to which such light beams has been converged.
The light beam converged on the signal recording surface is caused to undergo modulation in light quantity or light polarization direction in dependency upon the information signal recorded on the signal recording surface and is reflected by the signal recording surface. The reflected light thus obtained returns to the object lens.
The reflected light beam reflected by the signal recording surface is received by the photo-detector after passed through the object lens. This photo-detector is a light receiving element such as photo-diode, and serves to receive the reflected light beam which has been passed through the object lens to convert it into an electric signal. On the basis of the electric signal outputted from the photo-detector, reproduction of information signal recorded on the disc-shaped optical recording medium is carried out.
Moreover, on the basis of the electric signal outputted from the photo-detector, a focus error signal indicating distance in the optical axis direction of the object lens between the focal point of the light beam by the object lens and the signal recording surface, and a tracking error signal indicating distance in the radial direction of the disc-shaped optical recording medium between the focal point and the recording track on the signal recording surface are generated. The biaxial actuator is controlled on the basis of the focus error signal and the tracking error signal to allow the object lens to undergo movement operation so that these respective error signals are converged into zero (0).
Meanwhile, since such a disc-shaped optical recording medium is used as an auxiliary memory (unit) for computer and/or recording medium for speech and picture signals, recording density of information signals is being caused to be high.
In order to carry out write and read operations of information signals with respect to the disc-shaped optical recording medium of which recording density has been caused to be high, it is necessary to allow the object lens to have greater numerical aperture (NA), and to allow the wavelength of light emitted of the light source to be shorter to reduce (diameter of) beam spot formed as the result of the fact that the light beams are converged onto the disc-shaped optical recording medium.
However, when the numerical aperture of the object lens becomes greater, respective degrees of allowance of inclination of the disc-shaped optical recording medium, variations (unevenness) in thickness of the transparent base of the disc-shaped optical recording medium, and defocus (deviation in focal point) of the light beams on the disc-shaped optical recording medium are reduced. As a result, write and read operations of information signals with respect to the disc-shaped optical recording medium become difficult.
For example, when inclination (skew) with respect to the optical axis of the object lens of the disc-shaped optical recording medium takes place, wave front aberration in the light beam converged on the signal recording surface takes place, so electric signal (RF output) outputted from the photo-detector is affected.
In this wave front aberration, coma-aberration of the third order (cubic coma-aberration) produced in proportion to the third power of the numerical aperture of the object lens and approximately the first power of inclination angle (skew angle) of the disc-shaped optical medium is dominant. Accordingly, the allowed value with respect to the inclination of the disc-shaped optical recording medium is inversely proportional to third power of the numerical aperture of the object lens. Namely, when the numerical aperture becomes great, the allowed value becomes small.
In optical discs (disc like the so-called "compact discs") caused to be of structure including a disc-shaped transparent base (substrate) having thickness of 1.2 mm and diameter of 80 mm or 120 mm and formed by polycarbonate, and widely used in general at present, there are instances where inclination of .+-.0.5.degree. to .+-.1.degree. takes place.
When the numerical aperture (NA) of the object lens is increased in such a optical disc, wave front aberration as described above takes place in light beams irradiated onto the optical disc. As a result, beam spot on the optical disc takes asymmetrical form, and interference between codes remarkably takes place. Thus, precise signal reproduction becomes difficult.
The quantity of such cubic coma-aberration is proportional to the thickness of the transparent base (substrate) of the optical disc. For this reason, the thickness of the transparent base is thinned (e.g., is caused to be 0.6 mm), thereby making it possible to reduce the cubic coma-aberration to one half. In the case where it is assumed that coma-aberration is caused to be decreased in this way, optical discs including the transparent base having thickness of 1.2 mm and optical discs including the transparent base having thickness of 0.6 mm would be used in mixed state as the above-mentioned optical disc.
Meanwhile, when plane parallel plate having thickness t is inserted into an optical path of a converged light beam converged by the object lens, there takes place spherical aberration proportional to t.times.(NA).sup.4 in relation to the thickness t and numerical aperture NA of the object lens.
The object lens is designed so that this spherical aberration is corrected. Namely, since quantity of the spherical aberration produced also varies when the thickness of the transparent base changes, the object lens is designated as an object lens caused to be in conformity with the thickness of a predetermined transparent base.
Further, in the case where an attempt is made such that, e.g., object lens designed so as to become in conformity with an optical disc including a transparent base having thickness of 0.6 mm is used to carry out recording and reproduction of information signals with respect to optical disc including transparent base having thickness of 1.2 mm (e.g., "compact disc", write once type optical disc, or magneto-optical disc), such difference of thickness between these transparent bases (0.6 mm) would be greatly beyond the allowed range of error of thickness of the transparent base with which the optical pick-up can cope. In this case, the object lens cannot correct spherical aberration taking place resulting from difference of thickness of the transparent base, thus failing to carry out satisfactory recording and reproduction of information signals.
For this reason, an optical pick-up provided with two object lenses is conventionally proposed as disclosed in the Japanese Patent Application No. 354198/1995. As shown in FIG. 29, this optical pick-up is caused of the structure in which a first object lens 105 and a second object lens 106 are attached on a single lens bobbin 104 of a biaxial actuator 103. In this optical pick-up, light beams emitted from a light source 107 are incident to any one of the first and second object lens 105, 106 through a collimator lens 111 and a mirror 112. The first and second object lenses 105, 106 have numerical aperture values different from each other. Further, the biaxial actuator 103 is disposed on an optical system block 108 within which the light source 107 is included.
In the disc player caused to be of the structure including this optical pick-up, an optical disc 101 of the first kind in which the thickness of the transparent base is, e.g., 0.6 mm, or an optical disc 102 of the second kind in which the thickness of the transparent base is, e.g., 1.2 mm is held by the disc table of which central portion is attached to the drive shaft of spindle motor (not shown), and is caused to undergo rotational operation. Further, the optical pick-up is supported so that it is permitted to undergo movement operation in the axial direction of a guide shaft 109 as indicated by arrow S in FIG. 29 by the guide shaft 109. This optical pick-up is caused to undergo movement operation in the radial direction of the optical disc 101 or 102 held on the disc table.
In this optical pick-up, when the optical disc 101 of the first kind is loaded on the disc table, the light source 107 is caused to be turned ON to carry out write and read operations of information signals with respect to the optical disc 101 of the first kind through the first object lens 105. On the other hand, when the optical disc 102 of the second kind is loaded on the disc table, the light source 107 is caused to be turned ON to carry out write and read operations of information signals with respect to the optical disc 102 of the second kind through the second object lens 106. Switching between the first and second object lenses 105, 106 on the optical path of light beams from the light source 107 is carried out by rotating the lens bobbin 104 about a support shaft 110 which supports this lens bobbin 104.
Further, the lens bobbin 104 of the biaxial actuator 103 is caused to undergo rotational operation about the support shaft 110 to thereby allow the respective object lenses 105, 106 to undergo movement operation in the tracking direction which is the direction substantially in parallel to the axial direction of the guide shaft 109 as indicated by arrow T in FIG. 29 to allow these object lenses 105, 106 to follow the recording tracks on the optical discs 101, 102.
However, in the write once type optical disc (so called "CD-R") including a transparent base having thickness of 1.2 mm, wavelength dependency at the time of read operation of information signals is high. For this reason, in the case where there is used light source of which wavelength of light emitted is caused to be short in order to allow the recording density of information signal to be high, read-out operation of information signals is impossible. Namely, the signal recording layer of the so-called "CD-R" is formed by material of organic pigment (coloring matter) system. For this reason, this signal recording layer absorbs light beams of which wavelength is caused to be short, e.g., light beams having wavelength of 635 nm to 650 nm so that the reflection factor is lowered. Accordingly, it is impossible to carry out read (read-out) operation of information signals by such light beams of which wavelength is caused to be short.
Accordingly, this invention has been proposed in view of the above-described actual circumstances, and its object is to provide an optical pick-up including two object lenses so that write and read operations of information signals can be satisfactorily carried out with respect to disc-shaped optical recording media in which thicknesses of their transparent bases are different from each other, wherein even in the case where either one of object lens is used, the optical pick-up is capable of carrying out satisfactory detection of information signals, and can be used also with respect to disc-shaped optical recording media in which wavelength dependency at the time of read operation of information signals is high.
In addition, another object of this invention is to provide a disc player which is provided with optical pick-up featured above, and which is capable of carrying out recording and reproduction of information signals with respect to disc-shaped optical recording media in which thicknesses of their transparent bases are different from each other, and is capable of using so far as disc-shaped optical recording media in which wavelength dependency at the time of read operation of information signals is high.